Red is one of the primary colours. It is a colour which symbolizes energy and heat. This is because it is also the colour of blood and fire. It also has the highest wavelength. This is why it is used for signs especially ones showing danger (as it can be seen from a huge distance).
Fun Fact: According to a test by a TV show, Red does not make bulls angry and cause them to attack (From which the popular saying ‘to see red’ comes from.) As a matter of fact, bulls (and most cattle) are colour blind. It’s actually a sudden movement which causes the bull to charge (like when a cape is being waved around.)